C# Preprocessor Directives - #If & # endif

最新推荐文章于 2025-08-19 14:07:58 发布

riverlau

最新推荐文章于 2025-08-19 14:07:58 发布

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分类专栏： Software Design 文章标签： preprocessor c# compilation properties compiler mvc

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本文详细解释了C#中#if编译指令的工作原理，包括如何使用符号进行条件编译，以及如何在不同场景下应用此功能。

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http://msdn.microsoft.com/en-us/library/4y6tbswk.aspx

When the C# compiler encounters an #if directive, followed eventually by an #endif directive, it will compile the code between the directives only if the specified symbol is defined. Unlike C and C++, you cannot assign a numeric value to a symbol; the #if statement in C# is Boolean and only tests whether the symbol has been defined or not. For example,

#define DEBUG
// ...
#if DEBUG
    Console.WriteLine("Debug version");
#endif

You can use the operators == (equality), != (inequality) only to test for true or false . True means the symbol is defined. The statement #if DEBUG has the same meaning as #if (DEBUG == true). The operators && (and), and || (or) can be used to evaluate whether multiple symbols have been defined. You can also group symbols and operators with parentheses.

namespace Mvc3RemoteVal.Controllers 
{
        public class HomeController : Controller
        {
            IUserDB _repository;

#if  InMemDB
            public HomeController() : this(InMemoryDB.Instance) { }
#else
            public HomeController() : this(new EF_UserRepository()) { }
#endif


            public HomeController(IUserDB repository) 
            {
                _repository = repository;
            }

            [...]
        }

In order to define a variable you could use the /define compiler option or use the Conditional compilation symbols in the Build tab of the properties of the project in Visual Studio: